The field of instrumentation, more particularly that of instrumentation for, for example, process control systems, utilizes an interface standard by which a 4 to 20 ma current loop control signal is used to communicate process information between devices. Some devices (e.g., process variable transmitters and control systems) send this current signal while other devices (e.g., valves and control systems) receive this signal.
In order to control the current in the loop, a variable impedance device (e.g., a transistor) is often utilized. This device is controlled such that the current in the loop is set to the desired value (e.g., 4-20 ma). Despite the particular components used to implement the circuit that controls the current, there is a finite probability that the variable impedance device therein may short, causing a drastic increase in the amount of current within the loop. If, a low impedance receiver is within the loop, an uncontrolled amount of current may flow, leading to both lost control and potential circuit damage. One example of a prior current transmission circuit is depicted in FIG. 1. Two floating power supplies, namely high-voltage supply 11 and low-voltage supply 13 power the circuit. High voltage supply 11 provides power for the current loop, while low-voltage supply 13 powers the components of the circuit.
The output current is produced by a digital to analog converter ("DAC"-selected from any type of DAC functional technology) 21 that drives a MOSFET 19, acting as a voltage to current converter (the MOSFET is the variable impedance device used to control output current). A voltage regulator 27 is connected to filter capacitor 29 and provide a voltage reference to DAC 21. Data is provided to DAC 21 through opto-couplers 25.
The output of DAC 21 drives MOSFET 19 (or other type of variable impedance element, e.g., a bipolar transistor) through a resistor 31 coupled to a filter capacitor 33 (to stabilize the output). Precision resistor 35 operates to provide a feedback signal for DAC 21.
A diode CR1 15 is optionally includable for use in redundant circuit configurations to provide secure signal clamping. In such a configuration, diode 15's anode can be tied to common in a redundant output-combining block. A diode CR2 17 provides against over voltage or reverse in the output circuit.
The circuit of FIG. 1 also includes a mechanism by which the processor controlling the circuit can read-back the present output. The read-back circuit includes analog to digital converter 23 (implemented using any type of A/D functional technology) which reads the output signal via resistor 37 and capacitor 39. Data and clocking is passed through opto-couplers 25 such that the processor controlling the system can read the current output.
The above-described circuit suffers from the aforementioned disadvantages in that a short in, e.g., MOSFET 19 will cause uncontrolled current flow, leading to both control loss and potential circuit damage. The present invention is directed toward solutions to these above-identified problems.